The present invention relates to a radial tire.
The global warming caused by discharge of carbon dioxide has recently proceeded, and the regulation of waste gas of an automobile and the regulation of the discharge of carbon dioxide has been rigidified. Further, in future, the depletion of diesel oil for an automobile obtained from petroleum resources such as gasoline and diesel is forecasted.
Accordingly, requests for fuel-economization for automobiles such as a truck and a bus has been strengthened more and more and development of a tire with little rolling resistance (a tire with little fuel consumption) has been emphasized.
As a method of reducing the rolling resistance of a tire, it is effective to improve tread occupying the maximum weight among tire members and the rolling resistance of a tire has been conventionally reduced by the improvement of the tread. However, since the tread is a tire member affecting abrasion resistance of the tire and grip performance most strongly, it approaches to technical limit to reduce the rolling resistance while keeping abrasion resistance and grip performance.
Consequently, it becomes important to reduce the rolling resistance of the tire by improving a tire member other than the tread from now on.
A breaker cushion or a side wall packing is mentioned as the tire member enabling the reduction of the rolling resistance by improving it without affecting the abrasion resistance of a tire and grip performance. Herein, the breaker cushion means the tire member 1 shown in FIG. 1 and is an important tire member greatly affecting the durability of the tire, ride quality and the like. Further, the side wall packing means the tire member 4 shown in FIG. 2, enhances the rigidity of a tire, as well as relaxes shear stress to enhance the durability of a bead portion.
As an improvement process of the breaker cushion or the side wall packing, for example, the proportion of a filler for reinforcement such as carbon black is reduced by increasing the proportion of a rubber component in a rubber composition for the breaker cushion or a rubber composition for the side wall packing, and the loss tangent tan δ of the rubber composition has been reduced. However, since reinforcement effect to the rubber composition is not adequately obtained and the complex elastic modulus E* of the rubber composition is lowered, the breaker cushion or the side wall packing is easily deformed and energy loss becomes large; therefore the reduction of the rolling resistance of the tire could not be attained.
Further, a large amount of a curing agent is added to the rubber composition for the breaker cushion or the rubber composition of the side wall packing, and the loss tangent tan δ of the rubber composition has been reduced. The rolling resistance can be reduced by adopting this procedure without reducing the proportion of a filler for reinforcement in the rubber composition, but the physical property change of the rubber composition by thermal degradation caused by tire running is enlarged and when the rubber composition is used as the breaker cushion, feature (fracture feature) which can endure impact to the side wall of a tire which is bestowed when a tire runs on a curbstone or when the tire runs on uneven road is not adequate. Further, when the rubber composition is used as the side wall packing, the tire is deformed by load applied to the tire; therefore shear strain generated between body plies is relaxed by deforming the tire by load applied to the tire and the fracture feature enough to endure the load applied to the tire is not adequately obtained.
The request level of the fracture feature differs depending on the kind of a tire, and a radial tire which is used for a vehicle such as a truck and a bus in which load is heavier than a light truck requires the superior fracture feature.
In order to improve the fracture feature of the rubber composition for the breaker cushion or the side wall packing which is used for a vehicle such as a truck and a bus, it has been known that the loss tangent tan δ is reduced and the complex elastic modulus E* and the fracture feature are improved in good balance by reducing the proportion of a filler for reinforcement in the rubber composition, using carbon black (N134 and the like in ASTM code of USA Material Test Association Planning (ASTM 1765)) having high reinforcing property and a large dibutyl phthalate oil absorption amount (DBP oil absorption amount). However, there is a problem that difference between a portion reinforced with carbon black and a portion not reinforced with it in the rubber composition becomes enlarged by developing an aggregate which is formed by linking the particles of carbon black, fracture from the portion not reinforced is easily generated; therefore the fracture feature is lowered. Further, it is difficult to adequately disperse carbon black in the rubber composition and there occurs a problem that kneading time and the kneading number of times is increased.
Further, it is known that the loss tangent tan δ is reduced by enhancing the DBP oil absorption amount and enlarging the particles of carbon black (decreasing iodine adsorption amount). However, there is also a problem that the fracture feature becomes lowered as a result.
Thus, it is difficult that the loss tangent tan δ of the rubber composition for the breaker cushion or the side wall packing is reduced and the complex elastic modulus E* and the fracture feature are improved in good balance, by only using carbon black as a filler for reinforcement.
As the filler for reinforcement, there is additionally silica and there can be obtained the effect that a rubber composition can be drastically reinforced by compounding silica and the loss tangent tan δ of the rubber composition is also reduced. However, it is difficult that the loss elastic modulus * necessary as the rubber composition for the breaker cushion or the side wall packing is not obtained unless a considerable amount of single silica is compounded and tan δ is adjusted to an objective value.
Consequently, not only silica but also carbon black are used in a combination as the filler for reinforcement.
For example, a rubber composition for the breaker cushion using carbon black with silica is disclosed in Japanese Unexamined Patent Publication No. 2004-161862, but carbon black which is the same as N330 class in ASTM code or a small code number. Even if silica is used in a combination, adequate effect was not obtained.
Thus, there is not obtained yet a rubber composition in which the loss tangent tan δ is reduced, and the complex elastic modulus E* and the fracture feature are improved in good balance by only taking the superior reinforcement effect of carbon black and silica.